Question

Need to find records in subinv that do NOT have a listing in subinv table with substatus 1
when there is a listing in inv with status 1.

There is only one record per part number in inv, but can be several records in subinv with one of several status numbers. My statement returns no rows, shows no errors, but there are records that qualify.

SELECT m.partnum        
  FROM inv m, 
       subinv s     
 WHERE m.status = '1'
   AND not exists (SELECT s.partnum     
                     FROM subinv s1
                    WHERE s1.substatus = '1')       

Answer 1

You have a Cartesian product in your outer query, which is not good (especially as you don’t want that join anyway), and you need to specify the part number in the EXISTS query:

SELECT m.partnum
  FROM inv AS m
 WHERE m.status = '1'
   AND NOT EXISTS
       (SELECT *
          FROM SubInv AS s1
         WHERE s1.substatus = '1'
           AND s1.partnum = m.partnum
       );

This features a correlated sub-query for the NOT EXISTS. It would be worth checking the performance of a filtered OUTER JOIN:

SELECT i.partnum
  FROM inv AS i
  LEFT JOIN subinv AS s ON s.partnum = i.partnum AND s.substatus = '1'
 WHERE i.status = '1'
   AND s.substatus IS NULL;

The LEFT JOIN condition will generate a NULL for s.substatus when there is no row with s.substatus = '1', and the WHERE clause only selects such rows. Not blindingly obvious as a transform, but it might be faster than the sub-query simply because it is a join rather than a sub-query that has to be executed for each row in the main inv table (that satisfies status = '1').